In petroleum processes used in refineries, it is frequently desired to achieve pressure control within a process unit by the use of variable orifice devices in the form of conventional slide valves. Sometimes in place of such slide valves fixed orifice discs are used in conjunction with slide valves to control the pressure of a process unit or the pressure of a flue gas being delivered to downstream equipment. When such fixed orifice discs or slide valves are in compressable fluid service, noise is frequently generated by such orifices and may present a problem. This often leads to vibration problems of components in the system due to resonance caused by the noise or sound energy generated by the variable or fixed orifice devices. By decreasing the pressure differential across an orifice, the noise level generated can frequently be reduced. Sometimes a number of orifices in series are used as a means for sound attenuation in combination with pressure control. The disadvantage of fixed orifices, however, is that pressure control and/or sound attenuation is achieved only at a constant flow rate. However, since petroleum process units' operating conditions are seldom uniform and therefore experience variable internal flow rates and pressure fluctuations, a variable orifice design is highly advantageous to accomplish optimum noise attenuation over a wide range of process conditions. Also, by providing a number of smaller fixed orifice openings in the discs, the frequency of the sound or noise generated can be increased so as to avoid or minimize vibration problems due to resonance effects. Such a variable orifice design as in applicant's present invention will insure good pressure control and/or effective sound attenuation in petroleum process units over a wide range of operating conditions. In particular, the present invention is such that it combines in a double disc variable orifice valve the provision of a fixed orifice as well as desirable frequency modification to control vibration or resonance problems. Accordingly, it is an object of the present invention to provide a simple and novel single valve design wherein a plurality of equally spaced, equally sized orifices are provided on adjacent orifice plates which can be rotated, one relative to the other.